Magnetic powder and isotropic bonded magnet

ABSTRACT

Disclosed herein is a magnetic powder which can provide magnet having excellent magnetic properties and having excellent reliability especially excellent heat stability. The magnetic powder is composed of an alloy composition represented by R x (Fe 1-a Co a ) 100-x-y-z-w B y M z  (where R is at least one kind of rare-earth element excepting Dy, M is at least one kind of element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9at%, y is 4.6-8.0at%, z is 0.1-3.0at%, and a is 0-0.30), and the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein when the magnetic powder is mixed with a binding resin and then the mixture is subjected to injection molding or extrusion molding to form a bonded magnet having a density ρ[Mg/m 3 ], the maximum magnetic energy product (BH) max [kJ/m 3 ] of the bonded magnet at a room temperature satisfies the relationship represented by the formula (BH) max/ρ   2 [x10 -9 J·m 3/g   2 ] ≧ 2.10, and the intrinsic coercive force H CJ  of the bonded magnet at the room temperature is in the range of 400-760 kA/m.

This application is a divisional application of U.S. Ser. No.09/755,641, filed Jan. 5, 2001, entitled “Magnetic Powder and IsotropicBonded Magnet” now U.S. Pat. No. 6,527,875.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic powder, a manufacturing methodof magnetic powder and bonded magnets. More particularly, the presentinvention relates to magnetic powder, a manufacturing method of themagnetic power and a bonded magnet which is produced, for example, usingthe magnetic powder.

2. Description of the Prior Art

For reduction in size of motors, it is desirable that a magnet has ahigh magnetic flux density (with the actual permeance) when it is usedin the motor. Factors for determining the magnetic flux density of abonded magnet include magnetization of the magnetic powder and thecontent of the magnetic powder to be contained in the bonded magnet.Accordingly, when the magnetization of the magnetic powder itself is notsufficiently high, a desired magnetic flux density cannot be obtainedunless the content of the magnetic powder in the bonded magnet is raisedto an extremely high level.

At present, most of practically used high performance rare-earth bondedmagnets use the isotropic bonded magnets which are made using MQP-Bpowder manufactured by MQI Inc. as the rare-earth magnetic powderthereof. The isotropic bonded magnets are superior to the anisotropicbonded magnets in the following respect; namely, in the manufacture ofthe bonded magnet, the manufacturing process can be simplified becauseno magnetic field orientation is required, and as a result, the rise inthe manufacturing cost can be restrained. On the other hand, however,the conventional isotropic bonded magnets represented by thosemanufactured using the MQP-B powder involve the following problems.

(1) The conventional isotropic bonded magnets do not have a sufficientlyhigh magnetic flux density. Namely, because the magnetic powder that hasbeen being used has poor magnetization, the content of the magneticpowder to be contained in the bonded magnet has to be increased.However, the increase in the content of the magnetic powder leads to thedeterioration in the moldability of the bonded magnet, so there is acertain limit in this attempt. Moreover, even if the content of themagnetic powder is somehow managed to be increased by changing themolding conditions or the like, there still exists a limit to theobtainable magnetic flux density. For these reasons, it is not possibleto reduce the size of the motor by using the conventional isotropicbonded magnets.(2) Although there are reports concerning nanocomposite magnets havinghigh remanent magnetic flux densities, their coercive forces, on thecontrary, are so small that the magnetic flux densities (for thepermeance in the actual use) obtainable when they are practically usedin motors are very low. Further, these magnets have poor heat stabilitydue to their small coercive forces.(3) The conventional bonded magnets have low corrosion resistance andheat resistance. Namely, in these magnets, it is necessary to increasethe content of the magnetic powder to be contained in the bonded magnetin order to compensate the low magnetic properties (magneticperformance) of the magnetic powder. This means that the density of thebonded magnet becomes extremely high. As a result, the corrosionresistance and heat resistance of the bonded magnet are deteriorated,thus resulting in low reliability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide magneticpowder that can produce a bonded magnet having excellent magnetizationand having excellent reliability especially excellent temperaturecharacteristics (that is, heat resistance and heat stability), andprovide an isotropic bonded magnet formed from the magnetic powder.

In order to achieve the above object, the present invention is directedto magnetic powder composed of an alloy composition represented byR_(x)(Fe_(1-y)Co_(y))_(100-x-z-w)B_(z)Al_(w) (where R is at least onekind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9at %, and w is 0.02-1.5 at %), the magnetic powder being constitutedfrom a composite structure having a soft magnetic phase and a hardmagnetic phase, wherein the magnetic powder has magnetic propertiescharacterized in that:

-   -   when the magnetic powder is formed into an isotropic bonded        magnet having a density ρ [Mg/m³] by mixing with a binding resin        and then molding it, the remanent magnetic flux density Br[T] at        the room temperature satisfies the relationship represented by        the formula of Br/ρ [×10−⁻⁶T·m³/g]≧0.125;    -   the irreversible susceptibility (χ_(irr)) of the isotropic        bonded magnet which is measured by using an intersectioning        point of a demagnetization curve in the J-H diagram representing        the magnetic properties at the room temperature and a straight        line which passes the origin in the J-H diagram and has a        gradient (J/H) of −3.8×10⁻⁶ H/m as a starting point is equal to        or less than 5.0×10⁻⁷H/m; and    -   the intrinsic coercive force (H_(CJ)) of the isotropic bonded        magnet at the room temperature is in the range of 320-720 kA/m.

According to the magnetic powder as described above, it is possible toprovide bonded magnets having excellent magnetic properties as well asexcellent heat resistance (heat stability) and corrosion resistance.

In the present invention, it is preferred that when the magnetic powderis formed into an isotropic bonded magnet by mixing with a binding resinand then molding it, the absolute value of the irreversible flux loss(initial flux loss) is equal to or less than 6.2%.

This makes it possible to provide bonded magnets having especiallyexcellent heat resistance (heat stability).

In these cases, it is preferred that said R comprises rare-earthelements mainly containing Nd and/or Pr. This makes it possible toimprove saturation magnetization of the hard phase of the compositestructure (in particular, nanocomposite structure), and thereby thecoercive force is further enhanced.

Further, it is also preferred that said R includes Pr and its ratio withrespect to the total mass of said R is 5-75%. This makes it possible toimprove the coercive force and rectangularity with less drop of theremanent magnetic flux density.

Further, it is also preferred that said R includes Dy and its ratio withrespect to the total mass of said R is equal to or less than 14%. Thismakes it possible to improve the coercive force and the heat resistance(heat stability) without markedly lowering the remanent magnetic fluxdensity.

In the present invention, it is also preferred that the magnetic powderis obtained by quenching the alloy of a molten state. According to this,it is possible to refine the microstructure (crystalline grains)relatively easily, thereby enabling to further improve the magneticproperties of the bonded magnet.

Further, it is also preferred that the magnetic powder is obtained bymilling a melt spun ribbon of the alloy which is manufactured by using acooling roll. According to this, it is possible to refine themicrostructure (crystalline grains) relatively easily, thereby enablingto further improve the magnetic properties of the bonded magnet.

Furthermore, it is also preferred that the magnetic powder is subjectedto a heat treatment for at least once during the manufacturing processor after its manufacture. According to this, homogeneity (uniformity) ofthe structure can be obtained and influence of stress introduced by themilling process can be removed, thereby enabling to further improve themagnetic properties of the bonded magnet.

In the magnetic powders described above, it is preferred that theaverage particle size lies in the range of 0.5-150 μm. This makes itpossible to further improve the magnetic properties. Further, when themagnetic powder is used in manufacturing bonded magnets, it is possibleto obtain bonded magnets having a high content of the magnetic powderand having excellent magnetic properties.

Another aspect of the present invention is directed to an isotropicbonded magnet formed by binding a magnetic powder containing Al with abinding resin, wherein the isotropic bonded magnet is characterized inthat:

-   -   when the density of the isotropic bonded magnet is defined as ρ        [Mg/m³], the remanent magnetic flux density Br[T] at the room        temperature satisfies the relationship represented by the        formula of Br/ρ [×10⁻⁶ T·m³/g]≧0.125;    -   the irreversible susceptibility (χ_(irr)) of the isotropic        bonded magnet which is measured by using an intersectioning        point of a demagnetization curve in the J-H diagram representing        the magnetic properties at the room temperature and a straight        line which passes the origin in the J-H diagram and has a        gradient (J/H) of −3.8×10⁻6 H/m as a starting point is less than        5.0×10⁻⁷H/m; and    -   the intrinsic coercive force (H_(CJ)) of the isotropic bonded        magnet at the room temperature is in the range of 320-720 kA/m.

According to the magnetic powder as described above, it is possible toprovide an isotropic bonded magnet having excellent magnetic propertiesas well as excellent heat resistance (heat stability) and corrosionresistance.

In this isotropic bonded magnet, it is preferred that said magneticpowder is formed of R-TM-B—Al based alloy (where R is at least onerare-earth element and TM is a transition metal containing Iron as amajor component thereof). This also makes it possible to provide anisotropic bonded magnet having particularly excellent magneticproperties as well as particularly excellent heat resistance (heatstability) and corrosion resistance.

Further, in this isotropic bonded magnet, it is also preferred that themagnetic powder is composed of an alloy composition represented byR_(x)(Fe_(1-y)Co_(y))_(100-x-z-w)B_(z)Al_(w) (where R is at least onekind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9at %, and w is 0.02-1.5 at %). This also makes it possible to provide anisotropic bonded magnet having particularly excellent magneticproperties as well as particularly excellent heat resistance (heatstability) and corrosion resistance.

Moreover, in this isotropic bonded magnet, it is also preferred thatsaid R comprises rare-earth elements mainly containing Nd and/or Pr.This makes it possible to improve the coercive force of the bondedmagnet.

In this case, it is preferred that said R includes Pr and its ratio withrespect to the total mass of said R is 5-75%. This makes it possible toimprove the coercive force and rectangularity with less drop of theremanent magnetic flux density.

Further, it is also preferred that said R includes Dy and its ratio withrespect to the total mass of said R is equal to or less than 14%. Thismakes it possible to improve the coercive force and the heat resistance(heat stability) without markedly lowering the remanent magnetic fluxdensity.

In the isotropic bonded magnets as described above, it is preferred thatthe average particle size of the magnetic powder lies in the range of0.5-150 μm. This makes it possible to obtain an isotropic bonded magnethaving a high content of the magnetic powder and having excellentmagnetic properties.

Further, in the isotropic bonded magnets as described above, it is alsopreferred that the absolute value of the irreversible flux loss (initialflux loss) is equal to or less than 6.2%. This makes it possible toprovide particularly excellent heat resistance (heat stability).

Furthermore, in the isotropic bonded magnets as described above, it isalso preferred that the magnetic powder is constituted from a compositestructure having a soft magnetic phase and a hard magnetic phase. Thisimproves magnetizability and heat resistance (heat stability), thusleading to less deterioration in the magnetic properties with elapse oftime.

Preferably, the isotropic bonded magnets as described above are to besubjected to multipolar magnetization or have already been subjected tomultipolar magnetization. According to this, satisfactory magnetizationcan be made even in the case where sufficient magnetizing magnetic fieldis not obtained, thereby enabling to obtain sufficient magnetic fluxdensity.

Further, preferably, the isotropic bonded magnets as described above areused for a motor. By using the bonded magnet to motors, it becomespossible to provide small and high performance motors.

These and other objects, structures and advantages of the presentinvention will be apparent from the following detailed description ofthe invention and the examples taken in conjunction with the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration which schematically shows one example of acomposite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

FIG. 2 is an illustration which schematically shows one example of acomposite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

FIG. 3 is an illustration which schematically shows one example of acomposite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

FIG. 4 is a perspective view showing an example of the configuration ofan apparatus (melt spinning apparatus) for manufacturing a magnetmaterial.

FIG. 5 is a sectional side view showing the situation in the vicinity ofcolliding section of the molten metal with a cooling roll in theapparatus shown in FIG. 4.

FIG. 6 is a J-H diagram (coordinate) for explaining the irreversiblesusceptibility.

FIG. 7 is a J-H diagram (coordinate) that represents demagnetizationcurves and recoil curves.

DETAILED DESCRIPTION OF THE INVENTION

In the following, embodiments of the magnetic powder and the isotropicbonded magnet according to this invention will be described in detail.

General Description of the Invention

At present, a magnet having high magnetic flux density is practicallyrequired in order to reduce the size of motors or other electricaldevices. In a bonded magnet, factors that determine the magnetic fluxdensity are the magnetization of magnetic powder and the content(compositional ratio) of the magnetic powder contained in the bondedmagnet. When the magnetization of the magnetic powder itself is not sohigh, a desired magnetic flux density cannot be obtained unless thecontent of the magnetic powder in the bonded magnet is increased to anextremely high level.

As described in the above, the MQP-B powder made by MQI Inc. which isnow being widely used can not provide sufficient magnetic flux densitydepending on its use. As a result, in manufacturing the bonded magnets,it is required to increase the content of the magnetic powder in thebonded magnet, that is, it is required to increase the magnetic fluxdensity. However, this in turn leads to the lack of reliability in thecorrosion resistance, heat resistance and mechanical strength thereofand the like. Further, there is also a problem in that the obtainedmagnet has a poor magnetizability due to its high coercivity.

In contrast, the magnetic powder and the isotropic bonded magnetaccording to this invention can obtain a sufficient magnetic fluxdensity and an adequate coercive force. As a result, without extremelyincreasing the content of the magnetic powder in the bonded magnet, itis possible to provide a bonded magnet having high strength and havingexcellent moldability, corrosion resistance and magnetizability. Thismakes it possible to reduce the size of the bonded magnet and increaseits performance, thereby contributing to reduction in size of motors andother devices employing magnets.

Further, the magnetic powder of the present invention may be formed soas to constitute a composite structure having a hard magnetic phase anda soft magnetic phase.

While the MQP-B powder made by MQI Inc. is a single phase structure of ahard magnetic phase, the magnetic powder of the present invention hasthe composite structure which has a soft magnetic phase with highmagnetization. Accordingly, it has an advantage that the totalmagnetization of the system as a whole is high. Further, since therecoil permeability of the bonded magnet becomes high, there is anadvantage that, even after a reverse magnetic field has been applied,the demagnetizing factor remains small.

Alloy Composition of Magnetic Powder

Preferably, the magnetic powder according to this invention is formed ofR-TM-B—Al based alloys (where R is at least one rare-earth element andTM is a transition metal containing Iron as a major component thereof).Among these alloys, an alloy having alloy compositions represented byR_(x)(Fe_(1-y)Co_(y))_(100-x-z-w)B_(z)Al_(w) (where R is at least onekind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9at %, and w is 0.02-1.5 at %) is particularly preferred.

Examples of the rare-earth elements R include Y, La, Ce, Pr, Nd, Pm, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and a misch metal. In thisconnection, R may include one kind or two or more kinds of theseelements.

The content of R is set at 7.1-9.9 at %. When the content of R is lessthan 7.1 at %, sufficient coercive force cannot be obtained, andaddition of Al enhances the coercive force only to a small extent. Onthe other hand, when the content of R exceeds 9.9 at %, sufficientmagnetic flux density fails to be obtained because of the drop in themagnetization potential.

Here, it is preferable that R includes the rare-earth elements Nd and/orPr as its principal ingredient. The reason for this is that theserare-earth elements enhance the saturation magnetization of the hardmagnetic phase which constitutes the composite structure (especially,nanocomposite structure), and are effective in realizing satisfactorycoercive force as a magnet.

Moreover, it is preferable that R includes Pr and its ratio to the totalmass of R is 5-75%, and more preferably 20-60%. This is because when theratio lies within this range, it is possible to improve the coerciveforce (coercivity) and the rectangularity by hardly causing a drop inthe remanent magnetic flux density.

Furthermore, it is also preferable that R includes Dy and its ratio tothe total mass of R is equal to or less than 14%. When the ratio lieswithin this range, the coercive force can be improved without causingmarked drop in the remanent magnetic flux density, and the temperaturecharacteristic (such as heat stability) can be also improved.

Cobalt (Co) is a transition metal element having properties similar toFe. By adding Co, that is by substituting a part of Fe by Co, the Curietemperature is elevated and the temperature characteristic of themagnetic powder is improved. However, if the substitution ratio of Fe byCo exceeds 0.30, both of the coercive force and the magnetic fluxdensity tend to fall off. The range of 0.05-0.20 of the substitutionratio of Fe by Co is more preferable since in this range not only thetemperature characteristic of the magnetic powder but also the magneticflux density thereof are improved.

Boron (B) is an element which is important for obtaining high magneticproperties, and its content is set at 4.6-6.9 at %. When the content ofB is less than 4.6 at %, the rectangularity of the B-H (J-H) loop isdeteriorated. On the other hand, when the content of B exceeds 6.9 at %,the nonmagnetic phase increase and the magnetic flux density dropssharply.

Aluminum (Al) is an element which is advantageous for improving thecoercive force, and the effect of improvement of the coercive force isconspicuous when its content lies in the range of 0.02-1.5 at %. Inaddition, the rectangularity and the maximum magnetic energy product arealso improved in this range in accompanying with the improvement in thecoercive force, and the heat resistance and corrosion resistance alsobecome satisfactory. In this connection, however, when the content of Ris less than 7.1 at %, these effects due to addition of Al are verysmall as described above. Further, when the content of Al exceeds 1.5 at%, the drop in the magnetization occurs.

Further, another important effect obtained by containing 0.02-1.5 at %of Al is that the irreversible susceptibility (χ_(irr)) describedhereinafter can be made small and the irreversible flux loss can beimproved so that the heat resistance (heat stability) of the magnet isimproved. In this connection, it is to be noted that if the amount of Alis less than 0.02 at %, such effect is hardly realized and the effectfor improving coercive force described above is small.

Of course, Al itself is a known substance. However, in the presentinvention, it has found through repeatedly conducted experiments andresearches that by containing 0.02-1.5 at % of Al to the magnetic powderconstituted from a composite structure having a soft magnetic phase anda hard magnetic phase, the following four effects are realized, inparticular these four effects are realized at the same time, and this isthe significance of the present invention.

(1) The coercive force of the magnetic powder can be improved whilemaintaining the excellent rectangularity and the maximum magnetic energyproduct.

(2) The irreversible susceptibility (χ_(irr)) described below can bemade small.

(3) The irreversible flux loss can be improved, that is the absolutevalue thereof can be lowered.

(4) Better corrosion resistance can be maintained.

As described above, the significant feature of the present inventionresides in the addition of a trace amount of or a small amount of Al. Inthis regard, it is to be noted that addition of Al in the amount of morethan 1.5 at % leading to an adverse effect, thus it is out of the scopeof the present invention.

In this connection, the preferred range of the content of Al is 0.02-1.5at % as described above. In this case, it is more preferable that theupper limit of the range is 1.0 at %, and it is the most preferable thatthe upper limit is 0.8 at %.

In addition, for the purpose of further improving the magneticproperties, at least one other element selected from the groupcomprising Cu, Si, Ga, Ti, V, Ta, Zr, Nb, Mo, Hf, Ag, Zn, P, Ge, Cr andW (hereinafter, referred to as “Q”) may be contained as needed. Whencontaining the element belonging to Q, it is preferable that the contentthereof should be equal to or less than 2 at %, and it is morepreferable that the content thereof lies within the range of 0.1-1.5 at%, and it is the most preferable that the content thereof lies withinthe range of 0.2-1.0 at %.

The addition of the element belonging to Q makes it possible to exhibitan inherent effect of the kind of the element. For example, the additionof Cu, Si, Ga, V, Ta, Zr, Cr or Nb exhibits an effect of improvingcorrosion resistance.

Composite Structure

As described above, the magnetic material of the present invention has acomposite structure having a soft magnetic phase and a hard magneticphase.

In this composite structure (nanocomposite structure), a soft magneticphase 10 and a hard magnetic phase 11 exist in a pattern (model) asshown in, for example, FIG. 1, FIG. 2 or FIG. 3, where the thickness ofthe respective phases and the particle diameter are on the order ofnanometers (for example, 1-100 nm). Further, the soft magnetic phase 10and the hard magnetic phase 11 are arranged adjacent to each other (thisalso includes the case where these phases are adjacent throughintergranular phase), which makes it possible to perform magneticexchange interaction therebetween. In this regard, it is to be notedthat the patterns illustrated in FIG. 1 to FIG. 3 are only specificexamples, and are not limited thereto. For example, the soft magneticphase 10 and the hard magnetic phase 11 in FIG. 2 are interchanged toeach other.

The magnetization of the soft magnetic phase readily changes itsorientation by the action of an external magnetic field. Therefore, whenthe soft magnetic phase coexists with the hard magnetic phase, themagnetization curve for the entire system shows a stepped “serpentinecurve” in the second quadrant of the B-H diagram. However, when the softmagnetic phase has a sufficiently small size of less than several tensof nm, magnetization of the soft magnetic body is sufficiently andstrongly constrained through the coupling with the magnetization of thesurrounding hard magnetic body, so that the entire system exhibitsfunctions like a hard magnetic body.

A magnet having such a composite structure (nanocomposite structure) hasmainly the following five features.

(1) In the second quadrant of the B-H diagram (that is, J-H diagram),the magnetization springs back reversively (in this sense, such a magnetis also referred to as a “spring magnet”).

(2) It has a satisfactory magnetizability, and it can be magnetized witha relatively low magnetic field.

(3) The temperature dependence of the magnetic properties is small ascompared with the case where the system is constituted from a hardmagnetic phase alone.

(4) The changes in the magnetic properties with the lapse of time aresmall.

(5) No deterioration in the magnetic properties is observable even if itis finely milled.

In the alloy composition described in the above, the hard magnetic phaseand the soft magnetic phase are respectively composed of the following,for instance.

The hard magnetic phase: R₂TM₁₄B system (where, TM is Fe or Fe and Co),or R₂(TM, Al)₁₄B system (or R₂(TM, Q)₁₄B system, or R₂(TM, Al, Q)₁₄Bsystem).

The soft magnetic phase: TM (α-Fe or α-(Fe, Co) in particular), or analloy phase of TM and Al, a composite phase of TM and B, or a compositephase of TM, B and Al (or these phases containing Q).

Manufacture of Magnetic Powders

As for the magnetic powders according to this invention, it ispreferable that they are manufactured by melt-spinning (quenching) amolten alloy, and more preferable that they are manufactured by millinga melt spun (quenched) ribbon obtained by quenching and solidifying themolten metal of the alloy. An example of such a method will be describedin the following.

FIG. 4 is a perspective view showing an example of the configuration ofan apparatus (melt spinning apparatus) for manufacturing a magnetmaterial by the melt spinning (quenching) method using a single roll,and FIG. 5 is a sectional side view showing the situation in thevicinity of colliding section of the molten metal with the cooling rollin the apparatus shown in FIG. 4.

As shown in FIG. 4, the melt spinning apparatus 1 is provided with acylindrical body 2 capable of storing the magnet material, and a coolingroll 5 which rotates in the direction of an arrow 9A in the figurerelative to the cylindrical body 2. A nozzle (orifice) 3 which injectsthe molten metal of the magnet material alloy is formed at the lower endof the cylindrical body 2.

In addition, a heating coil 4 is arranged on the outer periphery of thecylindrical body 2 in the vicinity of the nozzle 3, and the magnetmaterial in the cylindrical body 2 is melted by inductively heating theinterior of the cylindrical body 2 through application of, for example,a high frequency wave to the coil 4.

The cooling roll 5 is constructed from a base part 51 and a surfacelayer 52 which forms a circumferential surface 53 of the cooling roll 5.

The base part 51 may be formed either integrally with the surface layer52 using the same material, or formed using a material different fromthat of the surface layer 52.

Although there is no particular limitation on the material of the basepart 51, it is preferable that the base part 51 is formed of a metallicmaterial with high heat conductivity such as copper or a copper alloy inorder to make it possible to dissipate heat of the surface layer 52 asquickly as possible.

Further, it is preferable that the surface layer 52 is formed of amaterial with heat conductivity equal to or lower than that of the basepart 51. Examples of the surface layer 52 include a metallic thin layerof Cr or the like, a layer of metallic oxide and a ceramic layer.

Examples of the ceramics for use in the ceramic layer include oxideceramics such as Al₂O₃, SiO₂, TiO₂, Ti₂O₃, ZrO₂ Y₂O₃, barium titanate,and strontium titanate and the like; nitride ceramics such as AlN,Si₃N₄, TiN, and BN and the like; carbide ceramics such as graphite, SiC,ZrC, Al₄C₃, CaC₂, and WC and the like; and mixture of two or more ofthese ceramics.

The melt spinning apparatus 1 is installed in a chamber (not shown), andit is operated preferably under the condition where the interior of thechamber is filled with an inert gas or other kind of gas. In particular,in order to prevent oxidation of a melt spun ribbon 8, it is preferablethat the gas is an inert gas such as argon gas, helium gas, nitrogen gasor the like.

In the melt spinning apparatus 1, the magnet material (alloy) is placedin the cylindrical body 2 and melted by heating with the coil 4, and themolten metal 6 is discharged from the nozzle 3. Then, as shown in FIG.5, the molten metal 6 collides with the circumferential surface 53 ofthe cooling roll 5, and after the formation of a puddle 7, the moltenmetal 6 is cooled down rapidly to be solidified while dragged along thecircumferential surface 53 of the rotating cooling roll 5, therebyforming the melt spun ribbon 8 continuously or intermittently. A rollsurface 81 of the melt spun ribbon 8 thus formed is soon released fromthe circumferential surface 53, and the melt spun ribbon 8 proceeds inthe direction of an arrow 9B in FIG. 4. The solidification interface 71of the molten metal is indicated by a broken line in FIG. 5.

The optimum range of the circumferential velocity of the cooling roll 5depends upon the composition of the molten alloy, the wettability of thecircumferential surface 53 with respect to the molten metal 6, and thelike. However, for the enhancement of the magnetic properties, avelocity in the range of 1 to 60 m/s is normally preferable, and 5 to 40m/s is more preferable. If the circumferential velocity of the coolingroll 5 is too small, the thickness t of the melt spun ribbon 8 is toolarge depending upon the volume flow rate (volume of the molten metaldischarged per unit time), and the diameter of the crystalline grainstends to increase. On the contrary, if the circumferential velocity istoo large, amorphous structure becomes dominant. Further, in thesecases, enhancement of the magnetic properties can not be expected evenif a heat treatment is given in the later stage.

Thus obtained melt spun ribbon 8 may be subjected to at least one heattreatment for the purpose of, for example, acceleration ofrecrystallization of the amorphous structure and homogenization of thestructure. The conditions of this heat treatment may be, for example, aheating in the range of 400 to 900° C. for 0.5 to 300 min.

Moreover, in order to prevent oxidation, it is preferred that this heattreatment is preferable to be performed in a vacuum or under a reducedpressure (for example, in the range of 1×10−1 to 1×10⁻⁶ Torr), or in anonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas,helium gas or the like.

The melt spun ribbon (thin ribbon-like magnet material) 8 obtained as inthe above has a microcrystalline structure or a structure in whichmicrocrystals are included in an amorphous structure, and exhibitsexcellent magnetic properties. The magnetic powder of this invention isobtained by milling thus obtained melt spun ribbon 8.

The milling method of the melt spun ribbon is not particularly limited,and various kinds of milling or crushing apparatus such as ball mill,vibration mill, jet mill, and pin mill may be employed. In this case, inorder to prevent oxidation, the milling process may be carried out undervacuum or reduced pressure (for example, under a reduced pressure of1×10⁻¹ to 1×10⁻⁶ Torr), or in a nonoxidizing atmosphere of an inert gassuch as nitrogen, argon, helium, or the like.

The average particle size of the magnetic powder is not particularlylimited. However, in the case where the magnetic powder is used formanufacturing isotropic bonded magnets described later, in order toprevent oxidation of the magnetic powder and deterioration of themagnetic properties during the milling process, it is preferred that theaverage particle size lies within the range of 0.5 to 150 μm, morepreferably the range of 0.5 to 80 cm, and still more preferably therange of 1 to 50 μm.

In order to obtain a better moldability of the bonded magnet, it ispreferable to give a certain degree of dispersion to the particle sizedistribution of the magnetic powder. By so doing, it is possible toreduce the void ratio (porosity) of the bonded magnet obtained. As aresult, it is possible to raise the density and the mechanical strengthof the bonded magnet as compared with a bonded magnent having the samecontent of the magnetic powder, thereby enabling to further improve themagnetic properties.

Thus obtained magnetic powder may be subjected to a heat treatment forthe purpose of, for example, removing the influence of stress introducedby the milling process and controlling the crystalline grain size. Theconditions of the heat treatment are, for example, heating at atemperature in the range of 350 to 850° C. for 0.5 to 300 min.

In order to prevent oxidation of the magnetic powder, it is preferableto perform the heat treatment in a vacuum or under a reduced pressure(for example, in the range of 1×10⁻¹ to 1×10⁻⁶ Torr), or in anonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas,and helium gas.

Thus obtained magnetic powder has a satisfactory bindability with thebinding resin (wettability of the binding resin). Therefore, when abonded magnet is manufactured using the magnetic powder described above,the bonded magnet has a high mechanical strength and excellent thermalstability (heat resistance) and corrosion resistance. Consequently, itcan be concluded that the magnetic powder is suitable for themanufacture of the bonded magnet.

In the above, the melt spinning (quenching) method is described in termsof the single roll method, but the twin roll method may also beemployed. Besides, other methods such as the atomizing method which usesgas atomization, the rotating disk method, the melt extraction method,and the mechanical alloying method (MA) may also be employed. Since sucha melt spinning method can refine the metallic structure (crystallinegrains), it is effective for enhancing the magnetic properties,especially the coercive force or the like, of the bonded magnet.

Bonded Magnets and Manufacture Thereof

Next, the isotropic bonded magnets (hereinafter, referred to simply alsoas “bonded magnets”) according to this invention will be described.

Preferably, the bonded magnets of this invention is formed by bindingthe above described magnetic powder using a binding resin (binder).

As for the binder, either of a thermoplastic resin or a thermosettingresin may be employed.

Examples of the thermoplastic resin include polyamid (example: nylon 6,nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12, nylon6-12, nylon 6-66, nylon 6T and nylon 9T); thermoplastic polyimide;liquid crystal polymer such as aromatic polyester; poly phenylene oxide;poly phenylene sulfide; polyolefin such as polyethylene, polypropyleneand ethylene-vinyl acetate copolymer; modified polyolefin;polycarbonate; poly methyl methacrylate; polyester such as poly ethylenterephthalate and poly butylene terephthalate; polyether; polyetherether ketone; polyetherimide; polyacetal; and copolymer, blended body,and polymer alloy having at least one of these materials as a mainingredient. In this case, a mixture of two or more kinds of thesematerials may be employed.

Among these resins, a resin containing polyamide as its main ingredientis particularly preferred from the viewpoint of especially excellentmoldability and high mechanical strength. Further, a resin containingliquid crystal polymer and/or poly phenylene sulfide as its mainingredient is also preferred from the viewpoint of enhancing the heatresistance. Furthermore, these thermoplastic resins also have anexcellent kneadability with the magnetic powder.

These thermoplastic resins provide an advantage in that a wide range ofselection can be made. For example, it is possible to provide athermoplastic resin having a good moldability or to provide athermoplastic resin having good heat resistance and mechanical strengthby appropriately selecting their kinds, copolymerization or the like.

On the other hand, examples of the thermosetting resin include variouskinds of epoxy resins of bisphenol type, novolak type, andnaphthalene-based, phenolic resin, urea resin, melamine resin, polyester(or unsaturated polyester) resin, polyimide resin, silicone resin,polyurethane resin, and the like. In this case, a mixture of two or morekinds of these materials may be employed.

Among these resins, the epoxy resin, phenolic resin, polyimide resin andsilicone resin are preferable from the viewpoint of their specialexcellence in the moldability, high mechanical strength, and high heatresistance. In this case, the epoxy resin is especially preferable.These thermosetting resins also have an excellent kneadability with themagnetic powder and homogeneity (uniformity) in kneading.

The unhardened thermosetting resin to be used maybe either in liquidstate or in solid (powdery) state at the room temperature.

The bonded magnet according to this invention described in the above maybe manufactured, for example, as in the following. First, the magneticpowder, a binding resin and an additive (antioxidant, lubricant, or thelike) as needed are mixed and kneaded (warm kneading) to form a bondedmagnet composite (compound). Then, thus obtained bonded magnet compositeis formed into a desired magnet form in a space free from magnetic fieldby a molding method such as compaction molding (press molding),extrusion molding, or injection molding. When the binding resin used isa thermosetting type, the obtained green compact is hardened by heatingor the like after molding.

In these three types of molding method, the extrusion molding and theinjection molding (in particular, the injection molding) have advantagesin that the latitude of shape selection is broad, the productivity ishigh, and the like. However, these molding methods require to ensure asufficiently high fluidity of the compound in the molding machine inorder to obtain satisfactory moldability. For this reason, in thesemethods it is not possible to increase the content of the magneticpowder, namely, to make the bonded magnet having high density, ascompared with the case of the compaction molding method. In thisinvention, however, it is possible to obtain a high magnetic fluxdensity as will be described later, so that excellent magneticproperties can be obtained even without making the bonded magnet highdensity. This advantage of the present invention can also be extendedeven in the case where bonded magnets are manufactured by the extrusionmolding method or the injection molding method.

The content of the magnetic powder in the bonded magnet is notparticularly limited, and it is normally determined by considering thekind of the molding method and the compatibility of moldability and highmagnetic properties. More specifically, it is preferable to be in therange of 75-99.5 wt %, and more preferably in the range of 85-97.5 wt %.

In particular, in the case of a bonded magnet to be manufactured by thecompaction molding method, the content of the magnetic powder shouldpreferably lie in the range of 90-99.5 wt %, and more preferably in therange of 93-98.5 wt %.

Further, in the case of a bonded magnet to be manufactured by theextrusion molding or the injection molding, the content of the magneticpowder should preferably lie in the range of 75-98 wt %, and morepreferably in the range of 85-97 wt %.

The density ρ of the bonded magnet is determined by factors such as thespecific gravity of the magnetic powder contained in the magnet and thecontent of the magnetic powder, and void ratio (porosity) of the bondedmagnet and the like. In the bonded magnets according to this invention,the density ρ is not particularly limited to a specific value, but it ispreferable to be in the range of 5.3-6.6 Mg/m³, and more preferably inthe range of 5.5-6.4 Mg/m³.

In this invention, since the magnetic flux density and the coerciveforce of the magnetic powder are high, the bonded magnet formed from themagnetic powder provides excellent magnetic properties (especially, highmaximum magnetic energy product (BH)_(max)) even when the content of themagnetic powder is relatively low. In this regard, it goes withoutsaying that it is possible to obtain the excellent magnetic propertiesin the case where the content of the magnetic powder is high.

The shape, dimensions, and the like of the bonded magnet manufacturedaccording to this invention are not particularly limited. For example,as to the shape, all shapes such as columnar, prism-like, cylindrical(ring-shaped), circular, plate-like, curved plate-like, and the like areacceptable. As to the dimensions, all sizes starting from large-sizedone to ultraminuaturized one are acceptable. However, as repeatedlydescribed in this specification, the present invention is particularlyadvantageous in miniaturization and ultraminiaturization of the bondedmagnet.

Further, in view of the advantages described above, it is preferred thatthe bonded magnet of the present invention is subject to multipolarmagnetization has been magnetized so as to have multipoles.

The bonded magnet of this invention as described in the above hasmagnetic properties in which the irreversible susceptibility (χ_(irr))which is measured by using an intersectioning point of a demagnetizationcurve in the J-H diagram (that is, coordinate where the longitudinalaxis represents magnetization (J) and the horizontal axis representsmagnetic field (H)) representing the magnetic characteristics at theroom temperature and a straight line which passes the origin in the J-Hdiagram and has a gradient (J/H) of −3.8×10⁻⁶H/m as a starting point isequal to or less than 5.0×10⁻⁷ H/m, and the intrinsic coercive force(H_(CJ)) of the magnet at the room temperature is in the range of320-720 kA/m. Hereinafter, explanation will be made with regard to therelationship among the irreversible susceptibility (χ_(irr)), theintrinsic coercive force (H_(CJ)), the remanent magnetic flux density(Br) and the density (p).

Irreversible Susceptibility (χ_(irr))

As shown in FIG. 6, the irreversible susceptibility (χ_(irr)) is theparameter which is represented by the following formula (unit isHenry/m, which is represented by H/m in this specification), wherein agradient of a tangential line of the demagnetization curve at a certainpoint P on the demagnetization curve in the J-H diagram is defined bydifferential susceptibility (χ_(dif)) and a gradient of a recoil curvewhen the recoil curve from the point P is drawn with the demagnetizationfield being once reduced (that is, a gradient connecting the both endsof the recoil curve) is defined by reversible susceptibility (χ_(irr)).

Irreversible Susceptibility (χ_(irr))=differential susceptibility(χ_(dif))−reversible susceptibility (χ_(rev))

In this connection, it is to be noted that in the present invention, thepoint P is defined as an intersectioning point of the demagnetizationcurve and the straight line y which passes the origin in the J-H diagramand has a gradient (J/H) of −3.8 10⁻⁶H/m.

In the present invention, the reason why the upper limit value of theirreversible susceptibility (χ_(irr)) at the room temperature is definedas 5.0×10⁻⁷H/m is as follows.

As stated in the above, the irreversible susceptibility (χ_(irr))represents the changing ratio of the magnetization, which is notreturned even if the absolute value thereof is reduced once afterdemagnetization field is applied, with respect to the magnetic field.Accordingly, by restraining the irreversible susceptibility (χ_(irr)) toa relatively small value, it is possible to improve heat stability ofthe bonded magnet and especially to reduce the absolute value of theirreversible flux loss. Actually, within the range of the irreversiblesusceptibility (χ_(irr)) of the present invention, the irreversible fluxloss obtained when the bonded magnet is being left in the atmosphere of100° C. for one hour and then the temperature is lowered into roomtemperature is equal to or less than 5% in its absolute value, whichmeans that practically satisfactory heat resistance (in particular whenused in motors or the like), that is satisfactory heat stability can beobtained.

In contrast, when the irreversible susceptibility (χ_(irr)) exceeds5.0×10⁻⁷H/m, the absolute value of the irreversible flux loss isincreased, so that it is not possible to obtain satisfactory heatstability. Further, since the intrinsic coercive force (H_(CJ)) islowered and the rectangularity thereof becomes poor, use of the obtainedbonded magnet is limited to the case where the permeance coefficient(Pc) becomes large (e.g. Pc≧5). Furthermore, the lowered coercive forcereduces the heat stability.

The reason why the irreversible susceptibility (χ_(irr)) at the roomtemperature is defined as 5.0×10⁻⁷ H/m is described above. However, itis preferable that the value of the irreversible susceptibility(χ_(irr)) is as smaller as possible. Therefore, in the presentinvention, it is preferable that the irreversible susceptibility(χ_(irr)) is equal to or less than 4.5×10⁻⁷H/m, and it is morepreferable that the irreversible susceptibility (χ_(irr)) is equal to orless than 4.0×10⁻⁷ H/m.

Intrinsic Coercive Force (H_(CJ))

It is preferred that the intrinsic coercive force (H_(CJ)) of the bondedmagnet at room temperature is 320 to 720 kA/m, and 400 to 640 kA/m ismore preferable.

If the intrinsic coercive force (H_(CJ)) exceeds the above upper limitvalue, magnetizability is deteriorated and therefore satisfactorymagnetic flux density can not be obtained.

On the other hand, if the intrinsic coercive force (H_(CJ)) is lowerthan the lower limit value, demagnetization occurs conspicuously when areverse magnetic field is applied depending upon the usage of the motorand the heat resistance at a high temperature is deteriorated.Therefore, by setting the intrinsic coercive force (H_(CJ)) to the aboverange, in the case where the bonded magnet (cylindrical magnet inparticular) is subjected to multipolar magnetization, a satisfactorymagnetization can be accomplished even when a sufficiently highmagnetizing field cannot be secured, which makes it possible to obtain asufficient magnetic flux density, and to provide a high performancebonded magnet, especially a bonded magnet for motor.

Relationship Between Remanent Magnetic Flux Density (Br) and Density (ρ)

In the present invention, it is preferred that the following formula (I)is satisfied between the remanent magnetic flux density Br(T) and thedensity ρ (Mg/m³).0.125≦Br/ρ[×10⁻⁶T·m³/g]  (I)

In this connection, it is more preferable that the following formula(II) is satisfied between the remanent magnetic flux density Br(T) andthe density ρ (Mg/m³), and it is most preferable that the followingformula (III) is satisfied therebetween.0.128≦Br/ρ[×10⁻⁶T·m³/g]≦0.16  (II)0.13≦Br/ρ[×10⁻⁶T·m³/g]≦0.155  (III)

When the value of Br/ρ[×10⁻⁶T·m³/g] is less than the lower limit valueof the formula (I), it is not possible to obtain a sufficient magneticflux density unless otherwise the density of the magnet is increased,that is the content of the magnetic powder in the magnet is increased.However, this in turn leads to problems in that available moldingmethods are limited, manufacturing cost is increased, and moldability islowered due to a reduced amount of the binding resin. Further, when amagnetic flux density of a certain level is to be obtained, a volume ofthe magnet is necessarily increased, which results in difficulty inminiaturizing devices such as motors.

Maximum Magnetic Energy Product (BH)_(max)

In the present invention, it is preferable that the maximum magneticenergy product (BH)_(max) of the bonded magnet is equal to or greaterthan 60 kJ/m³, more preferably equal to or greater than 65 kJ/m³, andmost preferably in the range of 70 to 130 kJ/m³. When the maximummagnetic energy product (BH)_(max) is less than 60 kJ/m³, it is notpossible to obtain a sufficient torque when used for motors depending onthe types and structures thereof.

Irreversible Flux Loss

In the present invention, it is preferable that the absolute value ofthe irreversible flux loss (that is, initial flux loss) is equal to orless than 6.2%, it is more preferable that it is equal to or less than5.0%, and it is most preferable that it is equal to or less than 4%.This makes it possible to obtain a bonded magnet having excellent heatstability (heat resistance).

EXAMPLES

Hereinbelow, the actual examples of the present invention will bedescribed.

Example 1

Magnetic powders with alloy compositions(Nd_(0.7)Pr_(0.25)Dy_(0.05))_(8.5)Fe_(bal)Co_(7.0)B_(5.3)Al_(w) (thatis, various types of magnetic powders in which the content w of Al ischanged variously) were obtained by the method described below.

First, each of the materials Nd, Pr, Dy, Fe, Co, B and Al was weighed,and then they were cast to produce a mother alloy ingot, and a sample ofabout 15 g was cut out from the ingot.

A melt spinning apparatus 1 as shown in FIG. 4 and FIG. 5 was prepared,and the sample was placed in a quartz tube 2 having a nozzle 3 (circularorifice of which diameter is 0.6 mm) at the bottom. After evacuating theinterior of a chamber in which the melt spinning apparatus 1 is housed,an inert gas (Ar gas) was introduced to obtain an atmosphere withdesired temperature and pressure.

The cooling roll 5 of the melt spinning apparatus 1 is provided with asurface layer 52 on the outer periphery of the base part 51 made of Cu.The surface layer 52 is formed of ZrC and has a thickness of about 6 μm.

Then, the ingot sample in the quartz tube 2 was melted by high frequencyinduction heating. Further, the jetting pressure (difference between theinner pressure of the quartz tube 2 and the pressure of the atmosphere)and the circumferential velocity were adjusted to obtain a melt spunribbon.

Thus obtained melt spun ribbon was then coarsely crushed, and the powderwas subjected to a heat treatment in an argon gas atmosphere at 690° C.for 300 sec. In this way, the various types of magnetic powders eachhaving different contents w of Nb were obtained.

Next, for the purpose of adjustment of the particle size, each magneticpowder is milled by a milling machine in an argon gas atmosphere toobtain a magnetic powder having an average particle size of 60 μm.

To analyze the phase structure of the obtained magnetic powders, therespective magnetic powder was subjected to X-ray diffraction usingCu—Kα line at the diffraction angle of 20°-60°. From the thus obtaineddiffraction pattern, the presence of diffracted peaks of a hard magneticphase, R₂(Fe,Co)₁₄B phase, and a soft magnetic phase, α-(Fe,Co) phase,were confirmed. Further, from the observation result using atransmission electronmicroscope (TEM), the formation of a compositestructure (nanocomposite structure) was confirmed in each magneticpowder.

A composite (compound) for bonded magnet was prepared by mixing therespective magnetic powder with a polyamide resin (Nylon 12) and a smallamount of hydrazine antioxidant and lubricant, and then kneading themunder the temperature of 225° C. for 15 min. In this case, thecompounding ratio (mixing ratio by weight) of the magnetic powder withrespect to the polyamide resin was common to the respective bondedmagnets. Specifically, in each of the bonded magnets, the content of themagnetic powder was about 97 wt %.

Then, each of the thus obtained compounds was crushed to be granular.Then, the granular substance(particle) was weighed and filled into a dieof a press machine, and then it was subjected to a compaction molding(in the absence of a magnetic field) under the temperature of 215° C.and the pressure of 750 MPa, to obtain an isotropic bonded magnet of acolumnar shape having a diameter of 10 mm and a height of 7 mm. Next,the density ρ of each bonded magnet was measured by means of theArchimedes' method. The results thereof were shown in the Table 1.

Evaluation for Magnetic Properties and Irreversible Susceptibility(χ_(irr))

After pulse magnetization is performed for the respective bonded magnetsunder the magnetic field strength of 3.2 MA/m, magnetic properties(remanent magnetic flux density Br, intrinsic coercive force (H_(CJ)),and maximum magnetic energy product (BH)max) were measured using a DCrecording fluxmeter (manufactured and sold by Toei Industry Co. Ltd withthe product code of TRF-5BH) under the maximum applied magnetic field of2.0 MA/m. The temperature at the measurement was 23° C. (that is, roomtemperature).

As shown in FIG. 7, in the measured demagnetization curve of J-Hdiagram, a recoil curve having a starting point at an intersectioningpoint P between the demagnetization curve and a straight line whichpasses an origin and has a gradient of −3.8×10⁻⁶H/m was produced withthe magnetic field being once changed to zero and then being returnedthe original state, and then the gradient of the recoil curve (that is,the gradient of the straight line connecting the both ends of the recoilcurve) was obtained and then it was defined as the reversiblesusceptibility (χ_(irr)). Further, the gradient of a tangential line ofthe demagnetization curve at the intersectioning point P was obtainedand then it was defined as the differential susceptibility (χ_(dif)).The irreversible susceptibility (χ_(irr)) was obtained by the formula ofχ_(irr)=χ_(dif)−χ_(rev). The results of them are shown in the attachedTable 1.

Evaluation for Heat Resistance

Next, the heat resistance (heat stability) of each of the bonded magnets(each having the column shape having the diameter of 10 mm and theheight of 7 mm) was examined. The heat resistance was obtained bymeasuring the irreversible flux loss (initial flux loss) obtained whenthe bonded magnet was being left in the atmosphere of 100° C. for onehour and then the temperature was lowered to the room temperature, andthen it was evaluated. The results thereof are shown in the attachedTable 1. In this connection, it is to be noted that smaller absolutevalue of the irreversible flux loss (initial flux loss) means moreexcellent heat resistance (heat stability).

Total Evaluation

As seen from the attached Table 1, the isotropic bonded magnets (No. 2to No. 6) formed of the magnetic powders in which the content w of Al is0.02 to 1.5 at % and the irreversible susceptibility (χ_(irr)) is equalto or less than 5.0×10⁻⁷ H/m exhibit excellent magnetic properties(remanent magnetic flux density, intrinsic coercive force and maximummagnetic energy product) and have small absolute value of theirreversible flux loss, so that the heat resistance of these magnets ishigh and the magnetizability thereof is excellent.

As described above, according to the present invention, it is possibleto obtain bonded magnets having high performance and high reliability(especially, heat resistance). In particular, these high performancesare exhibited when the bonded magnets are used in motors.

Example 2

In the same manner as Example 1, magnetic powders with alloycompositions(Nd_(0.75)Pr_(0.20)Dy_(0.05))_(8.6)Fe_(bal)Co_(6.9)B_(5.4)Al_(1.0) wereobtained by the method described below.

A composite (compound) for bonded magnet was prepared by mixing therespective magnetic powder with a polyamide resin (Nylon 12) and a smallamount of hydrazine antioxidant and lubricant, and then kneading themunder the temperature of 200-230° C. for 15 min. In this case, thecontent of the magnetic powder to be contained in each of the bondedmagnets was variously changed to obtain seven types of compounds.

Among thus obtained compounds, the compounds having a relatively highcontent of the magnetic powder were crushed to be granular, and thenthey were subjected to a compaction molding (in the absence of amagnetic field), while the compounds having a relatively low content ofthe magnetic powder were crushed to be granular, and then they weresubjected to an injection molding (in the absence of a magnetic field),thereby forming bonded magnets.

In this connection, it is to be noted that each bonded magnet was formedinto a columnar shape having a diameter of 10 mm and a height of 7 mm.

Further, it is also to be noted that the compaction molding was carriedout by filing each granular substance (particle) into a die of a pressmachine and then it was subjected to a compaction molding under thetemperature of 210-220° C. and the pressure of 750 MPa. Further, theinjection molding was carried out under the conditions that the dietemperature at molding was 90° C. and the temperature inside theinjection cylinder was 230-280° C.

For each of thus obtained bonded magnets, magnetic properties thereofwere measured and heat resistance thereof was also tested in the samemanner as the Example 1. The results thereof are shown in the attachedTable 2.

Total Evaluation

As seen from the attached Table 2, the bonded magnets according to thepresent invention exhibit, over the wide range of the density ρ,excellent magnetic properties (remanent magnetic flux density Br,maximum magnetic energy product (BH)_(max), and coercive force (H_(CJ)))and have a small absolute value of the irreversible flux loss, so thatthe heat stability (heat resistance) of these magnets is also excellent.

In particular, the bonded magnets according to the present inventionexhibit excellent magnetic properties even in the case where the bondedmagnets are low density bonded magnets (that is, bonded magnets having asmall content of the magnetic powder) which can be obtained by means ofan injection molding. The reason of this is supposed as follows.

When bonded magnets are low density, that is bonded magnets have arelatively large content of the binding resin, fluidity of the compoundduring the kneading process or molding process is high. This makes itpossible to knead the magnetic powder and the binding resin at arelatively low temperature within a short time period, so that it ispossible to easily accomplish that the magnetic powder and the bindingresin are uniformly mixed during the kneading process. Further, such ahigh fluidity of the compound makes it possible to easily carry out themolding at a relatively low temperature within a short time period. Inother words, molding conditions can be moderated. As a result, itbecomes possible to hold the deterioration (e.g. oxidization) of themagnetic powder during the kneading process and molding process at theminimum level, which results in production of bonded magnets having highmagnetic properties as well as improvement of the moldability.

Further, the high fluidity of the compound makes it possible to lower avoid ratio of the obtained bonded magnets, so that mechanical strengthand magnetic properties thereof are also improved.

Example 3

Using the magnetic powders obtained by Example 1, cylindrical(ring-shaped) isotropic bonded magnets having outer diameter of 22 mm,inner diameter of 20 mm and height of 4 mm were manufactured in the samemanner as Example 1. Then, thus obtained bonded magnets were subjectedto a multi-pole magnetization so as to have eight poles. At themagnetization process, an electric current of 16 kA was flowing througha magnetizing coil.

In this case, a magnitude of the magnetizing magnetic field forachieving 90% magnetizing ratio was relatively small, and this meansthat the magnetizability was excellent.

Further, using each bonded magnet as a magnet for a rotor, a spindlemotor for CD-ROM drive was assembled. Then, each of the DC motors wasrotated at 1000 rpm to measure a back electromotive force generated inthe coil winding thereof. As a result, it has been confirmed that avoltage equal to or less than 0.80V can be obtained in the motors usingthe bonded magnets of the samples No. 1 and No. 7 (ComparativeExamples), while a voltage equal to or greater than 0.96V which is morethan 20% higher value can be obtained in the motors using the bondedmagnets of the samples No. 2 to No. 6 (Example of present invention).

With this result, it has found that it is possible to manufacture highperformance motors by using the bonded magnets of the present invention.

In addition to the above, bonded magnets same as those of Examples 1 to3 were manufactured excepting that they are formed by means of anextrusion molding (the content of the magnetic powder in each bondedmagnet was 92 to 95 wt %). Then, the performance of these bonded magnetswere examined. As a result, it has found that the same results can beobtained by the motors using the bonded magnets

Further, bonded magnets same as those of Examples 1 to 3 weremanufactured excepting that they are formed by means of an injectionmolding (the content of the magnetic powder in each bonded magnet was 90to 93 wt %). Then, the performance of these bonded magnets wereexamined. As a result, it has found that the same results can beobtained by the motors using the bonded magnets

As described above, according to the present invention, the followingeffects can be obtained.

Since each of the magnetic powders contains a predetermined amount of Aland has a composite structure having a soft magnetic phase and a hardmagnetic phase, they have high magnetization and exhibit excellentmagnetic properties. In particular, intrinsic coercive force andrectangularity thereof are improved.

The absolute value of the irreversible flux loss is small and excellentheat resistance (heat stability) can be obtained.

Because of the high magnetic flux density that can be secured by thisinvention, it is possible to obtain a bonded magnet with high magneticperformance even if it is isotropic. In particular, since magneticproperties equivalent to or better than the conventional isotropicbonded magnet can be obtained with a magnet of smaller volume ascompared with the conventional isotropic bonded magnet, it is possibleto provide a high performance motor of a smaller size.

Moreover, since a higher magnetic flux density can be secured, inmanufacturing a bonded magnet sufficiently high magnetic performance isobtainable without pursuing any means for elevating the density of thebonded magnet. As a result, the dimensional accuracy, mechanicalstrength, corrosion resistance, heat resistance (heat stability) and thelike can be improved in addition to the improvement in the moldability,so that it is possible to readily manufacture a bonded magnet with highreliability.

Since the magnetizability of the bonded magnet according to thisinvention is excellent, it is possible to magnetize a magnet with alower magnetizing field. In particular, multipolar magnetization or thelike can be accomplished easily and surely, and further a high magneticflux density can be obtained.

Since a high density is not required to the bonded magnet, the presentinvention is adapted to the manufacturing method such as the extrusionmolding method or the injection molding method by which molding at highdensity is difficult as compared with the compaction molding method, andthe effects described in the above can also be realized in the bondedmagnet manufactured by these molding methods. Accordingly, variousmolding method can be selectively used and thereby the degree ofselection of shape for the bonded magnet can be expanded.

Finally, it is to be understood that the present invention is notlimited to Examples described above, and many changes or additions maybe made without departing from the scope of the invention which isdetermined by the following claims.

TABLE 1 Irreversible ρ Br H_(cJ) (BH)_(max) Br/ρ χ_(Irr) Flux LossSample No. W (Mg/m³) (T) (kA/M) (kJ/m³) (×10⁻⁶T · m³/g) (×10⁻⁷ H/m) (%)1 (Comp. Ex.) 0 6.27 0.82 319 70.7 0.131 7.7 −7.0 2 (This Invention)0.04 6.26 0.87 390 100.1 0.139 4.9 −5.2 3 (This Invention) 0.1 6.32 0.90462 108.3 0.142 4.2 −4.5 4 (This Invention) 0.2 6.29 0.91 480 111.10.145 3.5 −3.9 5 (This Invention) 0.5 6.30 0.88 503 106.9 0.140 3.0 −3.56 (This Invention) 1.5 6.33 0.83 542 93.7 0.131 2.7 −3.1 7 (Comp. Ex.)2.2 6.31 0.76 538 76.2 0.120 3.8 −4.0

TABLE 2 Kneading Molding Irreversible Temp. Molding Temp. ρ Br H_(cJ)(BH)_(max) Br/ρ χ_(Irr) Flux Loss Sample No. (° C.) Method ° C.) (Mg/m³)(T) (kA/M) (kJ/m³) (×10⁻⁶T · m³/g) (×10⁻⁷ H/m) (%)  8 (This Invention)220 Injection 230 5.30 0.78 532 80.4 0.147 2.2 −2.4 Molding  9 (ThisInvention) 203 Injection 245 5.50 0.80 521 85.2 0.145 2.5 −2.7 Molding10 (This Invention) 211 Injection 260 5.67 0.82 513 89.8 0.144 2.8 −3.0Molding 11 (This Invention) 216 Injection 275 5.80 0.83 506 93.2 0.1432.9 −3.2 Molding 12 (This Invention) 220 Compaction 210 5.95 0.84 50197.3 0.141 3.1 −3.5 Molding 13 (This Invention) 224 Compaction 215 6.210.87 495 105.2 0.140 3.7 −4.0 Molding 14 (This Invention) 230 Compaction220 6.48 0.90 481 113.4 0.139 4.4 −4.6 Molding

1. An isotropic bonded magnet formed by binding a magnetic powder containing Al with a binding resin, wherein the isotropic bonded magnet is characterized in that: the magnetic powder is composed of an alloy composition resented by R_(x)(Fe_(1-x)Co_(y))_(100-x-z-w)B_(z)Al_(w) (where R is at least one rare-earth element, x is 7.1-9.9 at %, v is 0-0.30, z is 4.6-6.9 at %, and w is 0.02-1.5 at %), when the density of the isotropic bonded magnet is defined as ρ[Mg/m³], the remanent magnetic flux density Br[T] at room temperature satisfies the relationship represented by the formula of Br/ρ[×10⁻⁶T·m³/g]>0.125; the irreversible susceptibility (χ_(irr)) of the isotropic bonded magnet which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing point of a demagnetization curve in the J-H diagram representing the magnetic properties at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10⁻⁶H/m as a starting point is less than 5.0×10⁻⁷H/m; and the intrinsic coercive force (H_(CJ)) of the isotropic bonded magnet at the room temperature is in the range of 320-720 kA/m.
 2. The isotropic bonded magnet as claimed in claim 1 wherein said R comprises rare-earth elements containing Nd and/or Pr.
 3. The isotropic bonded magnet as claimed in claim 1, wherein said R includes Pr and the ratio of Pr with respect to the total mass of said R is 5-75%.
 4. The isotropic bonded magnet as claimed in claim 1, wherein said R includes Dy and the ratio of Dy with respect to the total mass of said R is equal to or less than 14%.
 5. The isotropic bonded magnet as claimed in claim 1, wherein the average particle size of the magnetic powder lies in the range of 0.5-150 μm.
 6. The isotropic bonded magnet as claimed in claim 1, wherein the absolute value of the irreversible flux loss (initial flux loss) is equal to or less than 6.2%.
 7. The isotropic bonded magnet as claimed in claim 1, wherein the magnetic powder is constituted from a composite structure having a soft magnetic phase and a hard magnetic phase.
 8. The isotropic bonded magnet as claimed in claim 1, wherein the isotropic bonded magnet is to be subjected to multipolar magnetization or has already been subjected to multipolar magnetization.
 9. The isotropic bonded magnet as claimed in claim 1, wherein the isotropic bonded magnet is used for a motor. 